In industry, vacuum metallurgical melting systems have been built and operated to produce high quality ingots of reactive or refractory metals and/or their alloys in a single operational process directly from raw materials. In some such systems, raw materials can be provided into an open-top and open-bottom mold, having an heating induction coil surrounding at least part of the mold. The raw materials (or feed material) can be metals such as titanium, zirconium, nickel, cobalt, and/or their alloys, and can be provided into a mold of a vacuum metallurgical system in solid or molten form. When rendered into molten form, these metals can be contaminated by the oxide refractories generally used to make induction melting crucibles; therefore, to avoid contamination, these metals are typically melted in water-cooled copper vessels, although this melting technique is only about 25% efficient thermally.
Relatively small cross-sectional, ingots, bars, and castings of reactive or refractory metals/alloys made with vacuum metallurgical melting systems are used throughout the aerospace, automotive, energy, and medical industries. They can be machined or forged into any number of shapes. They may be used as the feedstock to be drawn into wire or to be rendered into a powdered metal. Such small cross-sectional bars are typically made from larger ingots which are incrementally heated to high temperatures and then forged down into the desired size. The forging process can lead to considerable yield loss however—a 60-70% yield of usable metal is typical. This is due in part to deformation of the ends of the ingot after a number of forging steps. In addition, it can take months for an ingot to await its turn in queue to be forged. Still further, due to the relatively small surface-area-to-volume ratio of the large ingots and associated cooling rates, the grain size of the finished product may be larger or less homogeneous than desired or needed.
Parts made from powdered metals are increasingly common and desired. Powdered metals are usually formed by grinding, or by remelting and atomizing, an ingot or casting that has been cast from a molten material. The parts can then be produced by consolidating the powder either directly into a final shape, or into a preform that is then machined. In most uses, it is usually important that each powder particle be of the same composition. This can only be achieved by ensuring that the metal ingot or casting from which the powder is formed is homogeneous, which can in turn only be achieved if the molten metal from which the ingot or casting is made is homogeneous.
The most common method of ensuring homogeneity in the molten metal (and/or alloy) is to stir the molten metal prior to pouring the motel metal in a mold and/or during the period of time the molten metal is in a mold being cast as an ingot. Another method uses an induction coil, which is discussed in U.S. Pat. No. 6,006,821 to Haun et al., assigned to the Applicant and dated Dec. 28, 1999, which is hereby incorporated by reference. Alternative implementations of heating using a single power source with heating elements wirelessly connected in series are also discussed in U.S. patent application Ser. No. 14/031,008 to Lampson et al., assigned to the Applicant and filed on Sep. 18, 2013, which is hereby incorporated by reference.
Additional complications can arise from attempting to cast relatively larger ingots made of intermetallics such as titanium, zirconium, nickel, cobalt, aluminum and/or other metals in that such ingots can be prone to minor, major, and/or catastrophic mechanical failure. In some cases, as an ingot cools after being cast and withdrawn from a furnace, a temperature gradient can develop between the exterior/surface of the ingot and the interior/core of the ingot. With some metals and alloys, the rate of cooling and temperature gradient may be sufficiently divergent or extreme such that the ingot cracks, breaks, or shears away from itself, rendering the ingot unfit and unsafe for industrial use, or post-processing to render into a relatively smaller ingot.
For all these reasons, it is desirable to cast the ingots nearer to their desired final cross-sectional size, a feat which has heretofore not been accomplished for small cross-sectional ingots. It is further desirable to ensure that the ingots are as homogeneous as possible, for reasons apparent to those of ordinary skill in the art.